Inversion splitting in the m 2 fundamental band of 15 NH 2 D and 15 ND 2 H M. Elkeurti a,1 , L.H. Coudert a, * , J. Orphal a,2 , C.E. Fellows b , S. Toumi c a LISA, UMR 7583 du CNRS, Universités Paris 12 et Paris 7, 61 Avenue du Général de Gaulle, 94010 Créteil Cedex, France b Laboratório de Espectroscopia e Laser, Universidade Federal Fluminense, Campus da Boa Viagem, Niterói, RJ 24210-340, Brazil c Laboratoire d’Etude et de Recherche en Instrumentations et, Communications d’Annaba (LERICA), Université Mokhtar Badji, B.P. 12, 23000 Annaba, Algerie article info Article history: Received 30 November 2009 In revised form 19 March 2010 Available online 30 March 2010 Keywords: Inversion Large amplitude motion Non-rigid molecule Rotation–inversion coupling Infrared spectrum High resolution Partially deuterated species of ammonia abstract This paper is concerned with an analysis of the high resolution spectra of the fundamental m 2 band of both 15 NH 2 D and 15 ND 2 H. Absorption spectra were recorded in the 690–1150 cm 1 region with a Fourier transform spectrometer. The assignment of the m 2 band transitions and a theoretical analysis of the vibration–inversion–rotation energy levels involved with these transitions were performed for both spe- cies. The first determination of the spectroscopic constants relevant to the symmetric and antisymmetric upper inversion substates of the fundamental m 2 band was carried out. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Ammonia has long been a subject of interest as it was the first molecule for which tunneling effects were evidenced [1]. The large-amplitude motion in which hydrogen atoms are exchanged between two isoenergetic configurations across a potential barrier was called ‘‘inversion” and leads to a large tunneling splitting which was first observed in the m 2 band [2] and later in the micro- wave spectrum [3] of the normal species. Since these very first investigations of the ammonia spectrum, the deuterium and 15 N containing isotopic species have also been studied. Among these, the partially deuterated ones, NH 2 D and ND 2 H, are of special inter- est as they also undergo the large-amplitude inversion motion, but they display a first order rotation–inversion Coriolis coupling [4,5] unlike the symmetrical NH 3 and ND 3 species. The ground vibrational state of the two partially deuterated species with 14 N was investigated using microwave spectroscopy [5–9] as well as far infrared (FIR) spectra [10]. Their excited vibrational states have been studied using high-resolution infrared spectroscopy [11–18]. The ground vibrational state of the 15 N containing species 15 NH 2 D and 15 ND 2 H was analyzed later using microwave and FIR spectra [19] and the former species has been detected in the interstellar media [20]. However, no results concerning excited vibrational states of 15 NH 2 D and 15 ND 2 H are available. In this paper we present experimental and theoretical analyses of the m 2 band 3 of these two molecules. In Section 2, the experimental details and the method used for the line assignment are described. Section 3 deals with the vibration–rotation–inversion Hamiltonian used for the line position analyses presented in Section 4. 2. Experimental details and assignment Several infrared spectra were recorded from 690 to 1150 cm 1 with a Bruker IFS 120 Fourier transform spectrometer. A 25 cm long absorption cell was filled with different mixtures of 15 NH 3 and 15 ND 3 at total pressures ranging from 0.8 to 2 mbar. This al- lowed us to obtain several spectra with different values of the 15 NH 2 D to 15 ND 2 H ratio. For all spectra the maximum optical path length difference was 5 m leading to a theoretical resolution of 0:002 cm 1 . Line positions were calibrated using the wavenumbers given in Ref. [21] for 15 NH 3 . The experimental uncertainty on the line positions is expected to be on the order of 0:5  10 3 cm 1 . Fig. 1 shows a portion of one of the spectra characterized by a high value of the 15 NH 2 D to 15 ND 2 H ratio. 0022-2852/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2010.03.009 * Corresponding author. Address: Laboratoire Inter-universitaire des Systèmes Atmosphériques, Université Paris 12, 61 Avenue du Général de Gaulle, 94010 Creteil Cedex, France. Fax: +33 1 45 17 15 64. E-mail addresses: laurent.coudert@lisa.univ-paris12.fr, coudert.laurent@ wanadoo.fr (L.H. Coudert). 1 Permanent address: Laboratoire d’Etudes Physico-Chimiques (LEPC), Université Tahar Moulay de Saı ¨da, B.P. 138, ENASR, 20002 Saı ¨ da, Algerie. 2 New address: Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Karlsruhe, Germany. 3 This notation, not in accordance with the IUPAC recommendations, is chosen for simplification and for analogy with the case of NH 3 . m 4 should be used instead of m 2 . Journal of Molecular Spectroscopy 261 (2010) 101–118 Contents lists available at ScienceDirect Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms